Electrostatic printing



June 20, 1967 N. R. NAIL ELECTROSTATIC PRINTING Filed Oct. 18.1963

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Will/III; F I6 54 NELSON R. IVA/L INVENTOR u Arromvsrs Patented June 20,1967 New Jersey Filed Oct. 18, 1963, Ser. No. 317,201 2 Claims. (Cl.117-17.5)

This invention relates to electrostatic printing and more particularlyto a method of making a succession of prints from a printing master. Theinvention involves forming an electrostatic image on an insulating sheetoverlying a printing master and xerographically developing the image toproduce a visible print.

The printing process of this invention is a fast, inexpensive, andsimple way of producing copies. Further, the process employs a printingmaster which is inexpenslve and which can be easily produced. Forexample, this process can use certain xerographic orphotoconductographic prints as the printing master. Some xerographicprocesses, for example, use a copy sheet consisting of a photoconductiveinsulating layer overlying a flexible, conductive support, and such asheet must be used for each print. However, by the printing process ofthe present invention, a single print made on such a sheet can be usedas a printing master to produce additional copies on regular paper, atconsiderable saving in cost.

A process of electrostatic printing from an insulating photoconductivelayer carried on an electrically conducting support and having an imageof electrically conducting material is known, see the Jarvis patent US.2,972,304. However, in that process the photoconductive layer is chargedand developed and the toner particles transferred to a sheet of paper.An advantage of the process of the present invention is that theelectrostatic image is produced and developed on the receiving sheet.This eliminates the transfer step of the above-identified patent.Further, after several printing operations from the same printing masterare carried out according to the process of the patent, it may bedesirable to clean the printing master to remove any toner particleswhich may have deposited on the background areas. Such a problem doesnot arise in the present invention since no toner particles are everrequired to come into contact with the printing master.

The electrostatic printing process of the present invention relates toelectrophotography insofar as it can employ certain xerographic orphotoconductographic prints as printing masters. As described below, anydocument having certain characteristics can be employed in the processof this invention as a printing master.

It is an object of this invention to provide a fast, inexpensive, andsimple method of electrostatic printing.

It is another object of this invention to provide an electrostaticprinting process which can use a xerographic or photoconductographicprint as the printing master.

Any document having conductive image areas and nonconductive backgroundareas, or vice versa, can be used as the printing master in the presentinvention. Such documents can be made in various ways; the two preferredsystems are xerography and photoconductography.

According to the invention, such a printing master is used by placingover the master and in contact therewith, an insulating sheet, and bycorona charging the insulating sheet to produce thereon an electrostaticimage. This electrostatic image can be developed by any of the usualelectrostatic toning methods such as magnetic brush, liquid development,etc. The toner particles which form the visible image on the insulatingsheet can be fixed thereto or transferred to a permanent record medium,thus allowing the insulating sheet to be reused for making additionalprints.

As will be described in more detail below, certain xerographic andphotoconductographic prints which consist of a visible image on aphotoconductive layer overlying a conductive support are particularlywell suited for use as the printing master in the present invention.This is true for several reasons. First, such prints are relativelyinexpensive documents in and of themselves, and are particularlyinexpensive as far as printing masters go, for example, as compared tothe letter press plates and type characters used in electroprinting.Second, since the top layer of the print is photoconductive, it can beused as an insulator in the subject process, or it can be exposed toactinic radiation and used as the conductor in the subject process.Thus, by forming the toned image areas on the electrophotographic printwith either insulating or conducting particles, and by proper use of thephotoconductor as either an insulator or a conductor, theelectrophotographic print can be readily used as the printing master inthis invention.

The invention will be more fully understood from the followingdescription when read in connection with the accompanying drawings inwhich:

FIGS. lA-lC constitute a flow chart of the electrostatic printingprocess of the present invention;

FIG. 2 illustrates an explanation of the mechanism resulting in theproduction of the electrostatic image on the insulating sheet;

FIG. 3 illustrates the developing step in a preferred embodiment of theprocess shown in FIGS. lA-IC to be used instead of the step shown inFIG. 1B;

FIGS. 4A and 4B illustrate two steps in a modification of the processshown in FIGS. lA-lC;

FIGS. 5A-5C constitute a flow chart of one method for preparing aphotoconductographic print to be used as a printing master according tothe present invention;

FIGS. 6A6D constitute a flow chart of one method of preparing axerographic print to be used as a printing master according to thepresent invention; and

FIGS. 7 and 8 illustrate variations in the process shown in FIGS. lA-IC.

The electrostatic printing process of the invention is shown in FIGS.lA-lC. A printing master 10 having electrically conductive image areas12 and electrically insulating background areas 14 is positioned on aconductive support 16. The conductive support 16 can be an integral partof the printing master 10 or separate therefrom. An insulating sheet 18is positioned in overlying contact with the printing master 10 andcorona charged, for example, by means of a voltage source 20, a switch22, and corona charging element 24. The element 24 is shown connected tothe negative terminal of the voltage source 20, and the conductivesupport 16 and the positive terminal of the voltage source 20 areconnected to ground. This charging step produces an electrostatic imageon the insulating sheet 18.

An explanation of the probable mechanism for the production of thiselectrostatic image will now be given with reference to FIG. 2. Aphotoconductor 30 is shown having a conducting area 32 and an insulatingarea 34. Because of the conductivity of conductive area 32, electricalchargescan move freely to its upper surface from a conducting support36, in contrast to the situation regarding the insulating area 34. Whenan insulating sheet 38 is placed over the printing master 30 and coronacharged, its area directly over the conducting area 32 becomes part of'ahigh capacitance system in which the insulating sheet 38 is thedielectric. The area of the insulating sheet 38 over the insulating area34 becomes part of a low capacitance system whose dielectric is composedof the insulating sheet 38 and the insulating area 34 in series.Consequently, during charging the part of the insulating sheet 38 overthe conducting area 32 ac- J cumulates a higher density of charge thanthe remaining part of the insulating sheet 38, due to the fact that thesheet 38 will tend to be charged to a uniform potential. This results ina xerographically developable difference between the two areas. 7 I

Returning now to the description of the process of the invention shownin FIGS. lAlC: FIG. 1B shows the xerographic development of theelectrostatic image produced on the insulating sheet 18. This image maybe developed by any of the usual electrostatic toning methods. FIG. 1Bshows the image being cascade-developed by means of a hopper 4tdeveloper powder 42 and a collection bin Q4. The toner particles arethen fixed in place, by any of the known fixing methods, and theinsulating sheet stripped from the printing master to produce a finalprint 46, as shown in FIG. 1C. I

Various modifications of the above-described process are possible. Forexample, after an electrostatic image is produced on the insulatingsheet 18 by the step shown in FIG. 1A, the insulating sheet 18 can bestripped from the printing master it) and placed on a grounded plate48-, during development, as shown in FIG. 3. It has been found that thisgrounded plate system yields slightly better prints than does theprocess shown in FIGS. 1A1C.

In another variation of the process shown in FIGS. lA1C, after theelectrostatic image on the insulating sheet 18 has been developed, thetoner particles may be transferred to another sheet such as sheet 50, asshown in FIGS. 4A and 4B, which is to be the final record medium. Thetoner particles are then fixed to sheet 50 by any of the known methods,to produce the final print on sheet 50 as shown in FIG. 4B.

As stated earlier, any document having conducting image areas andnonconducting background areas, or vice versa, can be used as theprinting master in the present invention. Various methods of producingsuch a printing master are possible. However, in the preferred method ofthe invention, such printing masters are conveniently produced byxerography or photoconductography.

According to the photoconductographic embodiment of the invention, aprint is produced for use as the printing master which has metallicimage areas such as those deposited from solutions containing ions ofheavy metals such as silver, nickel, iron, etc. Variousphotoconductographic processes for producing such a print are known.FIGS. A5C illustrate one such method. In FIG. 5A, a photoconductivesheet 60 on a conductive support 62 is exposed through a negativetransparency 64 by means of a light source 66 and a lens 68, to producean imagewise conductivity pattern in the photoconductive sheet 60. Thesheet 60 is electrolytically developed, for example, by means of aviscose sponge 70 wetted with a developer solutiton, as shown in FIG.5B. The sponge 70 is held at a positive potential with respect to thesupport 62 by means of a voltage source 72. The resultingphotoconductographic print 74 shown in FIG. 5C can be used as theprinting master in the process shown in FIGS. lA-lC.

FIGS. 6A6C illustrate one method for the production of a xerographicprint useful as a printing master in this invention. A xerographic sheetconsisting of a photoconductive layer 80, such as zinc oxide in aresinous binder, on a conductive support 82, such as aluminum foil, isgiven a negative, uniform electrostatic charge by means of, for example,a corona charger 84, switch 86, and a voltage source 88. The conductivesupport 82 and the positive terminal of the voltage source 88 areconnected to ground. The xerographic sheet is then exposed to actinicradiation through a transparency 90 by means of a light source 92 and alens 94 to produce a corresponding electrostatic image on thexerogr-aphic sheet, as shown in FIG. 6B. This image may then bedeveloped by any of the known xerographic methods. One such method isshown in FIG. 6C. A hopper 96 is used to cascade a developer composition98 across the xerographic sheet from which the excess developer flowsinto a collection bin 100. The toner particles are then fixed to thexerographic sheet to form the final print 99, shown in FIG. 6D.

In the photoconductographic print the visible image consists ofconducting particles. In the xerog-raphic embodiments, however, theimage areas can be either conducting or insulating.

An example of a process for preparing a photoconductographic print is asfollows:

Example 1 A sheet having a layer of dye-sensitized photoconductive zincoxide in a resin binder on an aluminum foil backing as normally used inphotoconductographc processes was exposed for 10 seconds to 400 ft.c. oftungsten radiation incident upon a high contrast, line-copy, negativetransparency contacting the zinc oxide surface. Upon termination ofexposure, the imagewise conductivity pattern induced by the exposure tolight was electrolytically developed with a viscose sponge wetted withan aqueous solution of the formulation:

Solution A.-l4.62 g. of Z-hydroxyethylamino oligoethylene sulfide(n=l.42) dissolved in 500 cc. of distilled WEII'EEI'.

Solution B.17.0 g. of silver nitrate dissolved in 500 cc. distilledwater.

Solution B was slowly added to Solution A with rapid stirring. Int-othis was then dissolved 50 g. of magnesium acetate tetrahydrate and 5cc. of glacial acetic acid and the pH of the final solution was raisedto 6.7 by the dropwise addition of Z-diethyl-aminoethanol.

The sponge carrying this developer was held at a potential of 60 voltpositive, with respect to the aluminum foil backing of thephotoconductive layer. After development, the print surface was blotteddry with a photographic blotter, rinsed several seconds with distilledwater, and then again blotted dry.

With this developer, the electrolytic deposition on the conductive areasof the zinc oxide surface consists of visible image material of metallicsilver and/or silver sulfide.

A photoconductographic print having conducting image areas and made by aprocess such as described above is employed as the printing master inthe printing process of the present invention as described in thefollowing examples.

ExampleZ An insulating sheet of 0.001-inch polyester film,alcohol-treated to remove spurious charges, was placed over the print,and the polyester side of the sandwich was placed under a 9 kv. coronadischarge for 8 seconds. While still in the dark the polyester surfacewas cascadedeveloped with Xerox N-l developer, which has a positivecharge on the particles. The visible image was then fixed by the spraylacquer Fixatif (Eagle Pencil Company, Danbury, Conn.).

Example 3 The printing process of Example 2 was repeated except that thepolyester sheet was removed from the master and placed on a groundedplate during development. Prints of slightly better quality were thusproduced.

Example 4 The printing process of Example 2 was repeated except that thetoner particles were transferred before fixing, by the use of pressurerollers, to a sheet of plain paper and fixed thereto. The polyestersheet, after slight cleaning, was used to make another print by the sameprinting process.

Example5 The printing process of Example 2 was repeated except that aninsulating sheet of 0.0075-inch cellulose acetate was used in place ofthe 0.0Ul-inch polyester film. Prints of similar quality were produced.

Example 6 The printing process of Example 2 was repeated except that aninsulating sheet of 0.0035-inch crazed polystyrene was used in place ofthe polyester sheet. Prints of similar quality were produced.

I have found that prints of improved quality over those produced by theprocesses of the above examples can be produced by treating thephotoconductive layer to increase the difference between theconductivities of the image areas and the non-image areas. Thephotoconductive layer can be treated by dark-adapting it for a period ofabout 24 hours to decrease the conductivity of the photoconductor in thebackground areas. FIG. 7 shows another method of treating thephotoconductive layer of a photoconductographic print. An electricalresistance heater 119 connected to a battery 112 by a switch 111 is usedto heat the photoconductive layer 113 in the dark. Results are producedby this method of treatment which are similar to those produced by thedark-adapting treatment. A still further method of treatment is shown inFIG. 8. By means of a corona charger 120 connected to a voltage source122 by a switch 121, the photoconductive layer 123 of a print iselectrostatically charged with the polarity of charge that is later usedto charge the insulating sheet. This corona-charging treatment can beused in addition to the dark-adapting and the heating treatment, or ifthe charging is prolonged for a few seconds, it is s'ufiicient treatmentin and of itself.

The following examples illustrate two methods of preparing and usingxerographic prints as printing masters in the present invention.

Example 7 A xerographic sheet comprising a layer of zinc-oxidein-resinbinder (of the type normally used in xerography) coated on a conductivesupport was charged under a -9 Kv. corona discharge and exposed througha negative line-copy transparency. The electrostatic image was developedin a liquid developer containing negatively charged toner particles(Sleight and Hellmuth Tri Dim A lithographic ink) dispersed incyclohexane. During development, a conducting electrode behind thephotoconductor was biased positively with respect to a grounded facingelectrode, causing a patternwise deposition of toner. The toner depositappeared to render the underlying portion of the zinc oxide layerconducting. The resulting print was used as the printing master in aprinting process similar to that of Examples 2-6. Positive prints wereproduced.

Example 8 A xerographic sheet comprising a zinc-oXide-in-resinbindercoating on a conducting backing was charged under a 9 kv. coronadischarge and exposed through a negative line-copy transparency. Theelectrostatic image was developed with a dispersion of positivelycharged toner par- 6 ti-cles (Sleight and Hellmuth Yellow No. 3046lithographic ink) in cyclohexane to yield a toner deposit correspondingto the charge pattern. This toner deposit appeared to render theunderlying portion of the zinc oxide layer insulating.

This Xerographic print was then used in a printing process similar tothat of Examples 2-6, except that the operation was carried out inroomlight. In this case the background areas had been renderedinsulating while the image areas had become conducting through exposureto roomlight. Positive prints were obtained.

The xerographic masters can be prepared with photoconductors other thanzinc oxide, e.g., selenium, sulfur, and anthracene.

The invention has been described in detail with particular reference topreferred embodiments thereof but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention as described hereinabove and as defined in the appendedclaims.

I claim:

1. I11 a process of electrostatic printing on an electrically insulatingreceiving sheet from a printing master comprising a photoconductivelayer on an electrically conductive support, said photoconductive layerhaving relatively electrically conductive and relatively electricallyinsulating areas which form an image, the steps comprising:

positioning said receiving sheet with one surface thereof in overlyingcontact with said photo-conductive layer;

electrostatically charging said receiving sheet from the other surfacethereof to a uniform potential to produce thereon an electrostaticimage;

transferring said receiving sheet, after said charging step,

to a surface of an electrically conductive grounded plate; and

xerographically applying toner particles to said electrostatic image onsaid receiving sheet to produce a visible image.

2. The process according to claim 1 including the step of treating saidphotoconductive layer prior to the step of positioning said receivingsheet, to increase the electrostatic contrast between said electricallyconductive and electrically insulating areas.

References Cited UNITED STATES PATENTS 2,297,691 10/ 1942 Carlson.2,357,809 9/1944 Carlson. 2,647,464 8/ 1953 Ebert. 2,972,304 2/ 1961Jarvis. 3,145,655 8/1964 Hope et al. 3,194,674 7/ 1965 Sakurai.

DAVID KLEIN, Primary Examiner.

1. IN A PROCESS OF ELECTROSTATIC PRINTING ON AN ELECTRICALLY INSULATINGRECEIVING SHEET FROM A PRINTING MASTER COMPRISING A PHOTOCONDUCTIVELAYER ON AN ELECTRICALLY CONDUCTIVE SUPPORT, SAID PHOTOCONDUCTIVE LAYERHAVING RELATIVELY ELECTRICALLY CONDUCTIVE AND RELATIVELY ELECTRICALLYINSULATING AREAS WHICH FORM AN IMAGE, THE STEPS COMPRISING: POSITIONINGSAID RECEIVING SHEET WITH ONE SURFACE THEREOF IN OVERLYING CONTACT WITHSAID PHOTO-CONDUCTIVE LAYER; ELECTROSTATICALLY CHARGING SAID RECEIVINGSHEET FROM THE OTHER SURFACE THEREOF TO A UNIFORM POTENTIAL TO PRODUCETHEREON AN ELECTROSTATIC IMAGE; TRANSFERRING SAID RECEIVING SHEET, AFTERSAID CHARGING STEP, TO A SURFACE OF AN ELECTRICALLY CONDUCTIVE GROUNDEDPLATE; AND XEROGRAPHICALLY APPLYING TONER PARTICLES TO SAIDELECTROSTATIC IMAGE ON SAID RECEIVING SHEET TO PRODUCE A VISIBLE IMAGE.